<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.2.0/jquery.js"></script>
</head>

<body>

</body>
<script>
    (function ($) {
        $.alert = function (msg) {
            alert(msg);
        }
       /*  $.fn.alert = function (msg) {
            alert(msg);
        } */
    })($);

    $.alert('你好')
</script>

</html>